After startup or during roaming of user equipment (UE), a primary task is to select a public land mobile network (PLMN) and successfully register with the PLMN, and only in this way, the UE can obtain communications services such as calling and data transmission.
After startup, the UE first attempts to register with a public land mobile network with which the UE successfully registered with last time (Registered PLMN, RPLMN for short). If the registration fails, the UE performs full-band network searching. The UE searches for all networks according to frequency bands and radio access technologies (RAT) that are supported by the UE, and then a list of available networks is formed, where each element in the list includes a PLMN and a RAT of an available network. Duration of the full-band network searching performed by the UE mainly depends on factors such as a frequency band and a RAT that are supported by the UE, a quantity of coverage cells in an area in which the UE is located, and software and hardware restrictions of the UE.
To facilitate faster registration of the UE with an available network, the UE selects a PLMN in a list of found available networks according to a specific rule. It is stipulated in the 3rd Generation Partnership Project (3GPP) specifications that UE selects a PLMN in the following sequence: home public land mobile network (Home PLMN, HPLMN for short)→user controlled public land mobile network (User PLMN, UPLMN for short)→operator controlled public land mobile network (Operator PLMN, OPLMN for short)→other available PLMNs ( ). For example, the UE sets, according to signal quality of the HPLMN, UPLMN, OPLMN, and other available PLMNs, priorities for available networks in the list of the found available networks, and sorts the available networks in descending order of the priorities; and the UE selects available networks in sequence according to the priorities and attempts to register, until the registration is successful.
For each selected available network, a registration process of the UE involves multiple rounds of network-wide signaling message exchange; moreover, when one registration attempt fails, the UE needs to continue to attempt to register with a same available network until a preset stop condition is satisfied, for example, a quantity of failed attempts reaches a preset quantity-of-failed-attempts threshold; therefore, a relatively long delay is caused when the UE attempts to register with an available network. In addition, in a scenario in which the UE is roaming, if there is a large quantity of operators in a roaming area of the UE, a list of available networks is long, for example, there are many small operators in Hong Kong, America, and some European countries, and the list of available networks may include dozens of elements (PLMN, RAT).
In the prior art, when UE initially accesses a network in a roaming area, it needs to take a relatively long time for the UE to successfully register with a network in the roaming area, which affects user experience due to an excessively long waiting time.